The present invention intends to provide an actuating device for an electrical power wrench wherein the output shaft has a plate mounted thereto which is radially impacted by a pin controlled by the eccentric force of the power wrench. The pin extends and radially impacts the plate when the desired eccentric force of the base member driven by a motor is reached. The present invention has arisen to mitigate and/or obviate the disadvantages of the conventional power wrenches.